This application is a National Stage of International Application No. PCT/EP2004/011587 filed Oct. 15, 2004, which claims the benefit of U.S. Provisional Application No. 60/511,673, filed Oct. 17, 2003.
1. Field of the Invention
The invention relates to a catadioptric projection objective for projecting a pattern arranged in the object plane of the projection objective into the image plane of the projection objective.
2. Description of the related Prior Art
Such projection objectives are used in microlithography projection exposure systems in order to produce semiconductor components and other finely structured components. Their purpose is to project patterns of photomasks or lined plates, which will be generically referred to below as masks or reticles, onto an substrate coated with a photosensitive layer with maximal resolution on a reducing scale.
In order to generate finer and finer structures, it is then necessary on the one hand to increase the numerical aperture (NA) of the projection objective on the image side and, on that the other hand, to use shorter and shorter wavelengths, preferably ultraviolet light with wavelengths of less than about 260 nm, for example 248 nm, 193 nm or 157 nm.
Purely refractive projection objectives have primarily been used to date in optical lithography. They are distinguished by a mechanically quite simple, centered construction which has only a single optical axis. It is also possible to use object fields centered on the optical axes, which minimize the etendue to be corrected and simplify alignment of the objective.
The form of the refractive design, however, is substantially constrained by two elementary image errors: the chromatic correction and the correction of the Petzval sum (image field curvature).
If only one refractive material is used (generally SiO2 for 193 nm, CaF2 for 157 nm) then the opportunity to correct the chromatic errors is very restricted. Full achromatization cannot be carried out. The best design compromise is then achieved by selecting single-waist designs with a small first convexity a large second convexity.
The correction of the Petzval condition (image field planarization) imposes the characteristic waist structure on the objective and entails very large maximum lens diameters, which increase the blank mass (mass of the lens material parts needed for the lens production). Once the waist structure has been established, then mass-optimized designs are obtained by trying to match the maximum diameters to one another in the first and second convexities. But this conflicts with the correction of the transverse chromatic aberration.
Simpler correction of the Petzval condition and an opportunity for chromatic correction are achieved with catadioptric designs. Here, the Petzval correction is achieved by the curvature of the concave mirror, and the chromatic correction is achieved by the refractive power of the negative lenses in front of the concave mirror (for CHL) and the aperture position with respect to the concave mirror (CHV).
A disadvantage of the catadioptric design, however, is that it is necessary to operate either with off-axial object fields, that is to say an increased etendue (in systems with geometrical beam splitting) or with physical beam splitter elements, which generally cause problems with the polarization.
In off-axial catadioptric systems, the requirements for the optical design may be formulated as follows: (1) minimize the etendue, (2) configure the geometry of the folding (beam deviations or deflections) so that a mounting technique can be developed for it and (3) correct the Petzval sum and the chromatic aberrations together in the catadioptric mirror group.
In order to keep the etendue small, the folding of the design should in principal take place in the low-NA region (that is to say in the vicinity of the object, for example) and in the vicinity of openings (that is to say close to the reticle or a real intermediate image).
As the numerical aperture rises, however, the numerical aperture on the object side and therefore the distance of the first fold from the reticle also increase, so that the etendue becomes greater. The diameter of the concave mirror and the size of the folding mirror are also increased. This can lead to problems with availability of space.
This can be remedied by firstly projecting the reticle onto an intermediate image by a first refractive relay system and by forming the first fold in the vicinity of the intermediate image. Such a catadioptric system is disclosed in EP 1 191 378 A1. It has a catadioptric objective part with the concave mirror. The light travels from the object plane onto a deflecting (folding) mirror placed in the vicinity of the first intermediate image, from there to the concave mirror and from the latter, while producing a second real intermediate image in the vicinity of a second deflecting mirror, into a refractive objective part which projects the second intermediate image onto the image plane (wafer).
A system with a similar structure is disclosed in WO 03/036361 A.
A catadioptric projection objective with a long, multi-lens relay objective for generating a first intermediate image, a polarization beam splitter, a catadioptric objective part with a concave mirror for generating a second real intermediate image, and a refractive objective part for projecting the second intermediate image onto the image plane, is disclosed in U.S. Pat. No. 5,861,997.
A disadvantage of such systems, however, is that the second refractive part again introduces chromatically and Petzval-undercorrected elements which need to be compensated for in the catadioptric part.
Other catadioptric systems with two real intermediate images are disclosed in JP 2002-372668 and Patent U.S. Pat. No. 5,636,066.